Seal construction for a fuel cell electrolyser and process for making a fuel cell with same

ABSTRACT

A sealing structure in a fuel cell and/or an electrolyzer (particularly a solid-oxide fuel cell and/or a solid-oxide electrolyzer) is arranged between neighboring separator plates of a cell stack. The sealing structure is constructed in at least two layers, including at least one insulating layer and at least one sealing layer.

[0001] This application claims the priority of German Patent document DE103 021 24.8, filed 21 Jan. 2003.

FIELD OF THE INVENTION

[0002] This invention relates to a sealing structure for a fuel cell oran electrolyzer, to a method of producing the sealing structure, and toa fuel cell or an electrolyzer.

BACKGROUND OF THE INVENTION

[0003] A conventional fuel cell stack 1, shown in FIG. 3, has two ormore individual fuel cells 2 which are stacked above one another in themanner of a tower. Each fuel cell 2 has an electrolyte layer 3, acathode layer 4 arranged on one flat side of the electrolyte layer 3,and an anode layer 5 arranged on the other flat side of the electrolytelayer 3. For contacting a neighboring fuel cell 2, a contacting layer 6is disposed on the cathode layer 4.

[0004] In addition, each individual fuel cell 2 has first and secondseparator plates 7, 8 that bound a combustible-gas space 9, into whichthe anode layer 5 projects. The combustible-gas space 9 is connectedwith the anode layer 5 such that combustible gas, which flows throughthe combustible-gas space 9 (direction of the arrow 10), can come incontact with the free surface of the anode layer 5.

[0005] Between the second separator plate 8 of one fuel cell 2 and afirst separator plate 7 of the neighboring fuel cell 2, an oxidation gasspace 11 is constructed, through which oxidation gas can flow (directionof the arrow 12), so that oxidation gas can flow against the freesurface of the cathode layer 4, which projects into the oxidation gasspace 11.

[0006] One flat side of the contacting layer 6 is in contact with thecathode layer 4 while the other flat side contacts a flat side of afirst separator plate 7 of the neighboring individual fuel cell 2 (thelatter facing the oxidation gas space 11). By way of correspondingopenings 13 in the first and second separator plates 7 and 8, allcombustible-gas spaces 9 are connected with one another. In the areabetween a second separator plate 8 and a first separator plate 7 of aneighboring individual fuel cell 2, the combustible-gas spaces 9 areseparated in a gas-tight manner from the oxidation gas space 11 by meansof a sealing layer 14, so that a fuel feeding duct 15 and a removal duct16 for the reaction products are formed. Thus, combustible gas can befed to the combustible-gas spaces 9 in the direction of the arrow 18 andflows through these in the direction of the arrow 10. In this case, thecombustible gas is oxidized in a fuel cell 2 along the anode layer 5,and the reaction product can leave the fuel cell stack 1 again in thedirection of the arrow 19. By way of correspondingly constructed feedingand removal ducts, the oxidation gas, analogous to the combustible gas,is guided through the oxidation gas spaces 11.

[0007] The separator plates 7 and 8 of an above-described fuel cellstack 1 therefore, on the one hand, have the function of electricallyconnecting the individual fuel cells 2, which are disposed in series. Onthe other hand, they ensure the separation of combustible and oxidationgas. For this purpose, the separator plates 7 and 8 (also called bipolarplates or interconnector plates) are constructed of acombustible-gas-tight, oxidation-gas-tight and electronically conductivematerial, such as a chrome-containing alloy, ferritic steel orperovskite. In order to ensure a reliable separation of the oxidationgases and the combustible gases, it is required that, in each case,between the second separator plate 8 of a first fuel cell 2 and thefirst separator plate 7 of a neighboring fuel cell 2, the feeding duct15 as well as the product removal duct 16 be reliably sealed off fromthe oxidation gas space 11.

[0008] It is known from the state of the art to construct the sealinglayer 14, for example, of glass-ceramic solders, which are normallyapplied as pastes or etched foils before the assembling of a fuel cellstack 1 onto the relevant sealing surfaces of the separator plates 7, 8.

[0009] These sealing materials (glass-ceramic solders) normally used inthe case of solid-electrolyte fuel cells have two characteristicsinfluencing one another in opposite directions. The coefficient ofthermal expansion of the sealing material is clearly lower in comparisonto the coefficients of expansion of most materials used for the bipolarplates 7 and 8. During the rapid heating of the fuel cell stack 1, thismay result in thermally induced tension cracks in the sealing layer 14and thus in a failure of its sealing effect. This is particularlycritical in the case of solid-electrolyte fuel cells (the so-calledSOFC's—solid oxide fuel cells) which operate in the high-temperaturerange. Particularly for solid-electrolyte fuel cells, which are stressedby a frequent starting and switching-off of the operation, thisrepresents a problem which has not been satisfactorily solved.

[0010] From the state of the art, it is conventional to increase thecoefficient of expansion of the sealing materials by means of additions.However, these additions frequently lead to a reduction of the electricresistance of the sealing material at the typically high operatingtemperatures of a solid-electrolyte fuel cell. By way of the sealinglayer 14 between a second separator plate 8 and a first separator plate7 of two neighboring individual fuel cells 2, this results inundesirable leak currents which impair the electric efficiency of a fuelcell stack 1.

[0011] Another disadvantage of the sealing device known according toFIG. 3 of the state of the art is that the materials for the sealinglayer 14 have a compression behavior and/or shrinking behavior whichdiffers in comparison to the contacting layer 6, whereby, during themounting of the fuel cell stack 1, undesirable inaccuracies occur whichmay make a reliable contacting between the contacting layer 6 and anadjoining separator plate 7 doubtful. Furthermore, it is disadvantageousthat the providing of a suitable sealing layer 14 before the assemblingof the fuel cell stack 1 requires high expenditures and cost because,for example, a sealing agent strand has to be established or, in thecase of a foil-type construction of the sealing layer 14, the latter hasto be produced separately and has to be positioned or inserted beforethe assembling process.

[0012] The above-mentioned glass-ceramic solders have two seriousdisadvantages:

[0013] 1. The coefficient of thermal expansion of glass ceramics isclearly lower in comparison to the coefficients of expansion of mostmaterials (chrome alloys, ferritic steel, perovskite) used for thebipolar plates. During the rapid heating of the fuel cell stack, thismay result in thermally induced tension cracks in the sealings and thusin a failure of the sealing effect. This is particularly critical in thecase of a mobile use of the fuel cell stack, for example, in anauxiliary energy supply unit in an automobile.

[0014] 2. Glass-ceramic solders shrink during the joining process, thatis, during the pressing-together and the first heating to the operatingtemperature of 750-900° C., to approximately 40%-70% of their initialvolume. The entire stack therefore sinks together during the joiningprocess. In order to ensure the tightness of the stack, the porouselectric contacting layer 6 of the fuel cell (see FIG. 3) also has toshrink by the same thickness. The difficulty now consists ofcoordinating the shrinkage behavior of the sealing layer and of thecontacting layer. The pasty ceramic suspensions normally used for theelectric contacting shrink even at low temperatures and compact attemperatures higher than 400° C. In the case of glass-ceramic solders,the shrinking process starts only at temperatures >500° C. and isconcluded only at temperatures >750° C. The two processes therefore donot take place simultaneously and frequently result in gas leakages,lack of electric contacting or a fracture of the SOFC (solid oxide fuelcell) because of locally excessive contact pressure forces.

[0015] Based on the above-mentioned disadvantages of the glass-ceramicsolders, the development of an alternative inorganic sealing mass wascarried out. With respect to its coefficient of expansion, it is betteradapted to the used bipolar plate materials and has only minimalshrinkage during the joining process, so that the necessity of usingelectric contacting materials especially adapted in their shrinkagebehavior is eliminated. However, the disadvantage of this sealing pasteis an electric insulating capacity which is insufficient at theoperating temperature. When solid-electrolyte fuel cell stacks are used,this results in electric leak currents (short circuits) between theindividual bipolar plates and thus results in power losses in thesystem.

[0016] German Patent Document DE 19515457 C1 describes a sealingstructure for a fuel cell. The fuel cell has an electrolyte layer thatconsists of an electrolyte matrix saturated with an electrolyte and, inthe sealing area, the electrolyte matrix is constructed to be extendedbeyond the electrodes. In the sealing area, the electrolyte matrix issaturated by means of a material chemically related to the electrolyte,which material is firm at the working temperature of the fuel cell.However, the suggested solution relates to a so-called molten-carbonatefuel cell which has a molten electrolyte which is present in liquid formin an electrolyte matrix. In the case of this type of fuel cell, oneusually speaks of a wet-sealing area because the electrolyte which ismolten in its operating condition, forms a wet area in the edge regionwhich is to be sealed off. However, this solution cannot be transferredto a solid-electrolyte fuel cell since, in the case of such asolid-electrolyte fuel cell (SOFC: solid-oxide fuel cell), no so-calledwet electrodes or wet electrolytes exist, and thus the problem on whichGerman Patent Document DE 19515457 C1 is based does not occur as aresult of the type of construction.

[0017] German Patent Document DE 19960516 A1 describes a sealing devicefor a fuel cell. The fuel cell has an electrolyte membrane that isextended into the edge sealing area between two separator plates and atwo-layer rubber seal that is arranged on the electrolyte membrane. Forthe sealing structure, it is suggested that one layer be constructed ofa soft sponge rubber and the second layer be constructed of a harderrubber, such as silicone rubber or butyl rubber. This document relatesto a so-called low-temperature fuel cell with a polymer membraneelectrolyte. These so-called low-temperature fuel cells have operatingtemperatures which are in the range of between 60° C. and 800C. Becauseof their operating temperatures, such fuel cells cannot be compared witha solid-electrolyte fuel cell because normally solid-electrolyte fuelcells are operated in temperatures range of between 700 and 1,100° C.Because of the high operating temperatures of a solid-electrolyte fuelcell, the sealing device suggested in German Patent Document DE 19960516A1 can therefore not be transferred to a solid-electrolyte fuel cell.

[0018] Japanese Patent Document JP 10092450 shows a fuel cell stack withinsulating layers and sealing layers, arranged in layers and formed asseparate components.

SUMMARY OF THE INVENTION

[0019] It is an object of the invention to provide a sealing structurefor a fuel cell or an electrolyzer, particularly a solid-electrolytefuel cell, which is insensitive to thermo-mechanical tensions andsimultaneously ensures an electric (particularly an electronic)insulation; that is, an impermeability for electrons. Furthermore, thesealing structure according to the invention is to be producible in asimple and cost-effective manner, particularly in comparison to thestate of the art, without additional working steps. In addition, thecompressibility and/or the shrinkage characteristic of the sealingstructure is to be adapted to that of the contacting layer and thusprovide a facilitated and particularly more process-secure mounting.

[0020] This and other objects and advantages are achieved by a sealingstructure in a fuel cell or electrolyzer according to the invention. Inan embodiment, the sealing structure according to the invention isarranged between neighboring separator plates of a cell stack, thesealing structure being constructed in at least two layers and having atleast one insulating layer and at least one sealing layer, and whereinthe insulating layer is arranged on a carrier element. Additionalobjects and advantages are achieved by a method of producing a sealingstructure for a fuel cell or an electrolyzer according to the invention.In an embodiment, the method comprises:

[0021] producing at least one insulating layer on a carrier element; and

[0022] producing at least one sealing layer made of a sealing material,the sealing structure being arranged in a sealing area of a fuel cellstack.

[0023] Still other objects and advantages are achieved by a fuel cell orelectrolyzer according to the invention that comprises a sealingstructure as described above. Further advantageous embodiments of theinvention are indicated in the specification and claims below.

[0024] In one embodiment of the invention, in order to counter the lackof electric insulation capacity of certain sealing materials, an outerskin made of aluminum oxide (Al₂O₃) in a γ-modification is produced bymeans of a targeted oxidation process, either on the sealing surfaces ofthe bipolar plates themselves or on insulation plates additionallyinserted between the sealing surfaces of the bipolar plates. γ-Al₂O₃ hasa very high electric resistance and an excellent corrosion stability inoxidizing as well as in reducing media. In another embodiment,additional insulation elements can be used when the use ofγ-Al₂O₃-forming steel types as a bipolar material is not desirable, forexample, because of the restriction on the electric current conductionbetween the bipolar plate and the cells.

[0025] In an embodiment, the outer skin of aluminum oxide is produced bythe targeted oxidizing of steel plates with a high aluminum content(>2%, preferably >4.5%) at temperatures >900° C., preferably >1,050° C.In order to ensure that the coefficients of thermal expansion of thebipolar plates, the sealing devices and possibly the insulation platecorrespond, ferritic steel types with chrome contents of approximately20% can mainly be used (for example, the Material Numbers 1.4765 and1.4767). More generally, ferritic steel types with a chrome content fromabout 15% to about 28% can be used. Since these materials can becommercially obtained as steel strips in many different thicknesses, theinsulation plates, which may be used, can simultaneously have thefunction of a spacer between the individual bipolar plates which wastaken over in the state of the art by the sealing device itself. Thestrips are to be machined easily in a shaping manner (stamping,punching, cutting) and, in principle, can be shaped into anyform—adapted to the bipolar plate. During the joining of the fuel cellstack, they are sealed in on both sides between the bipolar plates.

[0026] In preferred embodiments of the invention, electric short andleak currents between the individual cell elements in the fuel cellstack are prevented. The application of an electrically insulatingelement between the bipolar plates of solid-electrolyte fuel cell stackspermits the use of sealing materials that are less than completelyelectrically insulating for separating and distributing the combustibleand oxidation gases. The option of using these sealing devices, whichare conductive at the SOFC operating temperature, permits a noveljoining concept of the fuel cell stack for which a high-expenditureadaptation, which is difficult to implement, of the shrinking behaviorof the sealing device and a porous electric contacting layer of thesolid-electrolyte fuel cell can be eliminated. As a result, the joiningprocess is significantly simplified.

[0027] Furthermore, the use of electrically conductive sealing materialspermits the use of materials which are better adapted to thecoefficients of thermal expansion of the bipolar plate materials, sothat the probability of a failure of the sealing function because offaster thermal cycles is reduced, such as in the use of thesolid-electrolyte fuel cell in a mobile auxiliary energy supply unit.

[0028] The use of the above-mentioned electric insulation layers will beparticularly advantageous when the latter can be produced in acost-effective manner from commercially available materials. Thisapplies to the use of pre-oxidized ferritic steel types, but not, forexample, to sintered ceramic insulation elements.

[0029] The two possible application sites of the Al₂O₃-insulationlayer—directly onto the bipolar plate or onto additional insulationplates—each have specific advantages. The direct oxidation of thebipolar surface requires no additional components in the stack, and thusalso the number of working operations during the joining of the fuelcell stack does not increase. Alternatively, when additional insulationelements are inserted, the combination of electric and mechanical tasksof the insulation elements has an advantageous effect. This combinationis achieved when the insulation element simultaneously takes over thespacer function between neighboring bipolar plates from the sealingmaterial and the sealing can thus be reduced to a minimalthickness—defined only by the sealing function.

[0030] Other objects, advantages and novel features of the presentinvention will become apparent from the following detailed descriptionof the invention when considered in conjunction with the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031]FIG. 1 is a schematic cross-sectional view of a fuel cell stackaccording to the invention having two individual fuel cells which have asealing structure according to the invention;

[0032]FIG. 2 is a schematic cross-sectional view of a second embodimentof a fuel cell stack according to the invention having two individualfuel cells which are equipped with a second embodiment of a sealingstructure according to the invention; and

[0033]FIG. 3 is a schematic cross-sectional view of a conventional fuelcell stack.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] A fuel cell stack 1 (FIG. 1) according to the invention has atleast two individual fuel cells 2, preferably a plurality of individualfuel cells 2, which are stacked above one another in a tower-typemanner. The individual fuel cells 2 have an electrolyte layer 3, acathode layer 4 and an anode layer 5 and are preferably constructed asso-called solid-electrolyte fuel cells (SOFC solid-oxide fuel cells).The anode layer 5 is optionally arranged on a supporting substrate layer(not shown). The free flat side of the cathode layer 4 is connected viaa contacting layer 6 with a first separator plate 7 (also called bipolarplate or interconnector plate) of a neighboring individual fuel cell 2.

[0035] The fuel cell stack 1 according to the invention has combustiblegas spaces 9 through which combustible gas can flow by way of acombustible gas feeding duct 15 in the direction of the arrows 18, 10,19. By way of a removal duct 16, excess combustible gas and reactionproducts can be removed. Likewise, the fuel cell stack 1 according tothe invention has oxidation gas spaces 11 through which oxidation gascan flow by way of suitable feeding and removal ducts (not shown). Thecontacting layer 6 has an electrically conductive and porousconstruction so that the oxidation gas can flow through the contactinglayer 6 in the direction of the arrow 12.

[0036] For separating the combustible gases from the oxidation gases, afuel cell stack 1 according to the invention has a sealing structure 20which has a multi-layer, particularly at least two-layer construction.

[0037] According to a first embodiment of the sealing structure 20according to the invention, the latter has a sealing layer 21 and aninsulating layer 22. The insulating layer 22 consists of a metal oxide,particularly an aluminum oxide (Al₂O₃) which, in a particularlypreferred manner, is constructed in the so-called □-modification. Al₂O₃in the □-modification has a particularly high electric resistance and anexcellent corrosion stability in oxidizing as well as in reducing media.

[0038] In an embodiment corresponding to FIG. 1, the insulating layer 22is arranged in all required sealing areas 25 between two neighboringindividual fuel cells 2 on a top side 26 of one of the bipolar plates 7,8, which top side 26 faces an oxidation space 11. The arrangement of thesealing layer 21 on a free surface of a bipolar plate 7, 8 isadvantageous particularly when the bipolar plates 7, 8 are produced froma steel material with a high aluminum content (>2%). In this case, thealuminum oxide insulating layer can be produced by a targeted,particularly local oxidizing of the bipolar plates 7, 8 in the sealingareas 25, preferably above 900° C. The sealing layer 21 is arranged on afree surface 22 a of the insulating layer 22, in which case it should beensured that the sealing layer 21 is connected with no more than one ofthe bipolar plates 7 or 8 and, at the other end, comes in contact onlywith the insulating layer 22.

[0039] The sealing layer 21 is constructed, for example, of aglass-ceramic solder which, by means of additions, is adapted to thecoefficients of thermal expansion of the separator plates 7, 8.Advantageously, the shrinkage behavior of the sealing layer 21 (whenheated) is adapted to the shrinkage behavior of the contacting layer 6.An increased conductibility of the sealing layer 21, optionally causedby suitable additions, at the operating temperature of the fuel cells 2,particularly in the temperature range of from 750° C. to 900° C., inwhich solid-electrolyte fuel cells (so-called SOFC—solid-oxide fuelcells) are normally operated, is easily acceptable because of thereliable electric insulation by the insulation layer 22.

[0040] For the gas-tight sealing-off of the feeding and removal ductsfor the oxidation gas, which are not shown in FIGS. 1 and 2, a sealingstructure 20 according to the invention is correspondingly arrangedbetween a first separator plate 7 and a second separator plate 8 of anindividual fuel cell 2, so that the burnable-gas spaces 9 are separatedfrom the oxidation-gas carrying ducts.

[0041] Preferably, the insulating layer 22 covers a larger area on theseparator plate 7 or 8, on which it is mounted, than is required by thesealing layer 21, so that it is ensured that no “electric bridge” can beformed by the material of the sealing layer 21 when the fuel cell stackis joined together.

[0042] In embodiments corresponding to FIG. 1, the invention providesthe advantage that an improved sealing and insulating effect can beachieved between two individual fuel cells 2 without requiringadditional mounting steps in comparison to previous mounting sequencesduring the mounting of the fuel cell stack 1. The formation of theinsulating layer 22, for example, of Al₂O₃, can take place in a simplemanner by an oxidation process during the production of the separatorplates in a fully automatic fashion. Thus, a process-secure and reliablemounting of a fuel cell stack 1 corresponding to FIG. 1 of the inventionis ensured.

[0043] In a second embodiment of the sealing structure of the inventioncorresponding to FIG. 2, the insulating layer 22 is arranged on acarrier layer 23, in which case a first sealing layer 21 a is arrangedbetween the insulating layer 22 and a neighboring separator plate 7, anda sealing layer 21 b is arranged between the carrier layer 23 and itsneighboring separator plate 8. Thus, in this embodiment, the sealingstructure 20 has at least four layers, having at least one carrier layer23, at least one insulating layer 22 and at least two sealing layers 21a, 21 b.

[0044] In this embodiment, the insulating layer 22 is constructed of thesame material as the insulating layer 22 of an embodiment correspondingto FIG. 1. The sealing layers 21 a, 21 b are preferably constructed ofthe same material as the sealing layer 21 of an embodiment correspondingto FIG. 1. The carrier layer 23 is, for example, a steel plate with ahigh aluminum content (>2%). In the case of such steel plates, attemperatures above 900° C., the insulating layer 22 can be produced ofaluminum oxide by a targeted oxidizing.

[0045] Ferritic steel types with chrome-contents of approximately 20%(for example, Material No. 1.4765, particularly with an aluminum contentof 5-6%; Material No. 1.4767, particularly with an aluminum content of4.5-5.5%) are particularly preferred. These materials are particularlysuitable for the construction of the carrier layer 23 when theuniformity or the correlation of the coefficients of thermal expansionof the bipolar plates 7, 8, the sealing devices of the sealing layer 21and the insulation layer 22 is to be achieved. The above-mentionedmaterials are also particularly preferred because they are commerciallyavailable as strips in many different thicknesses and can easily beprocessed in a shaping manner, for example, stamped, punched and cut.

[0046] If the carrier layer 23 is constructed of a steel plate, by meansof the suitable selection of the plate thickness, the carrier layer 23can advantageously carry out an additional function, specifically thefunction of a spacer between two neighboring individual fuel cells 2.

[0047] Thus, in a simple manner, the shrinkage of the entire sealingstructure 20 can be adapted to the shrinkage of the contacting layer 6.For example, when a sealing material for the sealing layers 21 a, 21 bis used which has a high shrinkage, the thickness of the carrier layer23 can be selected to be relatively large, so that the sum of theshrinkages of the sealing layers 21 a, 21 b corresponds the totalshrinkage of the contacting layer 6. In the case of a sealing materialwhich has only a very slight shrinkage, by selecting a thinner carrierlayer 23, the remaining sealing layer thickness of the sealing layers 21a, 21 b can be selected to be so large that the sum of the shrinkages ofthe sealing layers 21 a, 21 b corresponds to the total shrinkage of thecontacting layer 6.

[0048] The coordination of the individual layer thicknesses of the firstsealing layer 21 a, of the second sealing layer 21 b and of the carrierlayer 23 can be defined in a limited number of tests by a person skilledin the art in such a manner that, with respect to its shrinkagebehavior, the sealing structure 20 corresponds to that of the contactinglayer 6. When the shrinkage of the material, of which the contactinglayer 6 consists, is particularly low, for example, also the layerthickness of the sealing layers 21 a, 21 b can be minimized such thatthe sealing-material-specific minimal thickness defined only by thesealing function is adjusted. The electric insulation capacity of thesealing device therefore no longer has to be considered when selectingthe layer thickness.

[0049] The carrier layer 23 is particularly preferably constructed of apre-oxidized ferritic steel because the operation of the oxidizing ofthe carrier layer 23 can therefore be eliminated during the productionof the fuel cells.

[0050] The characteristics of embodiments corresponding to FIGS. 1 and 2can also be combined. Particularly, also in the case of a sealingstructure 20 corresponding to FIG. 1, an adaptation of the shrinkagecharacteristic can be used by inserting a carrier layer 23, whichoptionally has no Al₂O₃-layer, only for the function of a spacer.

[0051] The method according to the invention will be explained in detailin the following by means of examples.

[0052] For producing a sealing structure 20 according to the invention,an insulating layer 22 is applied to a carrier 7, 8, 23. In particularlypreferred embodiments, the carrier may, on the one hand, be one of theseparator plates 7, 8 and, on the other hand, the carrier layer 23.

[0053] In an embodiment, an insulating layer 22, particularly made ofAl₂O₃, preferably Al₂O₃ in the γ-modification, is mounted on thecarriers 7, 8, 23 in the sealing areas 25. In this case, the material ofthe carrier 7, 8, 23 is provided with the insulating layer 22 bytargeted oxidation.

[0054] It is particularly advantageous to use a material for the carrier7, 8, 23 which contains aluminum in a sufficiently large quantity,particularly a quantity >2%. Suitable materials are, for example,materials of the Numbers 1.4765 and 1.4767.

[0055] In this case, the targeted oxidation preferably takes placeat >900° C., particularly at temperatures >1,050° C.

[0056] According to a particularly preferred embodiment, after theproduction of the insulating layer 22 in the sealing areas 25 of theseparator plates 7, 8, the sealing layer 21, particularly in the form ofa sealing material strand, is fitted onto the insulating layer 22.

[0057] If the insulating layer 22 is mounted on a carrier layer 23,particularly a carrier plate, the sealing structure is produced in thata sealing device strand for forming a first sealing layer 21 a isapplied in the sealing area 25 of the bipolar plates 7 and 8respectively. Subsequently, the carrier layer 23 having the insulatinglayers 22 will be fitted onto the first sealing layer 21 a. In thiscase, a sealing medium strand for forming is second sealing layer 21 bis applied to the fitted-on carrier layer 23 again in the sealing areas25. The above-described layer sequence is arranged between twoneighboring bipolar plates 7, 8 of two neighboring individual fuel cells2 such that either feeding and removal ducts respectively for burnablegas or feeding and removal ducts respectively for oxidation gas areconstructed, the burnable gas ducts each being connected with burnablegas spaces 9, and the oxidation gas ducts being connected with oxidationgas spaces 11.

[0058] It is an advantage of the first described embodiment that, forthe mounting of a fuel cell stack, in comparison to the state of theart, no additional parts exist which have to be used and thus themounting is not made difficult, although an improved adaptation of theinsulating characteristics and of the expansion or shrinkagecharacteristics of the sealing structure, particularly a possibleadaption of the shrinkage and of the coefficient of thermal expansion ofthe sealing mass to the corresponding parameters of the contacting layer6 or of the separator plates 7, 8 is achieved.

[0059] In the second embodiment of the method according to theinvention, it is advantageous that, despite an additional mounting part(carrier layer 23 with the insulating layer 22), which additionally hasto be inserted during the mounting of the fuel cell stack 1, it can beachieved that the carrier layer 23, together with the insulating layer22, is incompressible and can therefore take over a spacer function. Inaddition, by varying the thickness ratios of the carrier layer 23 and ofthe sealing layers 21 a, 21, the shrinkage of the sealing structure 20can be adapted to the shrinkage of the contacting layer 6 with respectto the absolute end measurement as well as in its progress during theshrinkage. In a particularly preferred case, the thicknesses of thesealing layers 21 a, 21 b can be reduced so far that only the minimumthickness defined for the sealing function is present and thus a savingof the relatively expensive sealing medium used for constructing thesealing layers 21 a, 21 b can be achieved. It is particularlyadvantageous that the gap width of the sealing gap, which is to befilled with sealing material, can be reduced considerably and the riskof a failing of sealing device is therefore considerably reduced.

[0060] The foregoing disclosure has been set forth merely to illustratethe invention and is not intended to be limiting. Since modifications ofthe disclosed embodiments incorporating the spirit and substance of theinvention may occur to persons skilled in the art, the invention shouldbe construed to include everything within the scope of the appendedclaims and equivalents thereof.

What is claimed is:
 1. A sealing structure for a fuel cell or an electrolyzer, having at least first and second neighboring separator plates with a sealing structure arranged between said separator plates, wherein: said sealing structure comprises at least two layers, including at least one insulating layer and at least one sealing layer; and the insulating layer is arranged on a carrier element.
 2. A sealing structure according to claim 1, wherein the fuel cell or electrolyzer is one of a solid-oxide fuel cell and a solid-oxide electrolyzer.
 3. A sealing structure according to claim 1, wherein the insulating layer is a metal oxide.
 4. A sealing structure according to claim 1, wherein the insulating layer is made of Al₂O₃.
 5. A sealing structure according to claim 4, wherein the Al₂O₃ is present in the structure of the γ-modification.
 6. A sealing structure according to claim 1, wherein the sealing layer comprises an inorganic material.
 7. A sealing structure according to claim 6, wherein the inorganic material is a glass-ceramic solder.
 8. A sealing structure according to claim 1, wherein the sealing layer has additions which ensure that the sealing layer is adapted to the thermal expansion behavior of the material of a separator plate.
 9. A sealing structure according to claim 1, wherein the carrier element is a carrier layer.
 10. A sealing structure according to claim 9, wherein the carrier layer is a steel plate with an aluminum content greater than 2%.
 11. A sealing structure according to claim 9, wherein the carrier layer is a steel plate with an aluminum content greater than 4.5%.
 12. A sealing structure according to claim 9, wherein the carrier layer is constructed of a ferritic steel with a chrome content of approximately 20%.
 13. A sealing structure according to claim 9, wherein the carrier layer is constructed of a ferritic steel with a chrome content of from about 15% to about 28%.
 14. A sealing structure according to claim 9, wherein the carrier layer is composed of Material Number 1.4765 or 1.4767.
 15. A sealing structure according to claim 1, wherein the carrier element is a separator plate, the insulating layer being arranged in the sealing areas.
 16. A sealing structure according to claim 1, wherein one or more carrier elements are provided with an insulating layer formed by pre-oxidation in at least one sealing area.
 17. A sealing structure according to claim 1, wherein the sealing structure is arranged in a fuel cell stack, the fuel cell stack being constructed of a plurality of individual fuel cells which are stacked above one another in a tower-type manner.
 18. A sealing structure according to claim 17, wherein the plurality of individual fuel cells have an electrolyte layer, a cathode layer and an anode layer, the anode layer being arranged on a carrying substrate layer.
 19. A sealing structure according to claim 17, wherein the insulating layer is arranged between two neighboring individual fuel cells on a top side of a separator plate, wherein the top side faces an oxidation space.
 20. A sealing structure according to claim 1, wherein the insulating layer is arranged between two neighboring separator plates of an individual fuel cell in the region of the sealing areas on at least one of the separator plates.
 21. A sealing structure according to claim 1, wherein the sealing layer is arranged on a free surface of the insulating layer.
 22. A sealing structure according to claim 1, wherein the sealing layer is constructed of an inorganic material containing additions that is adapted to the coefficients of thermal expansion of the separator plates.
 23. A sealing structure according to claim 1, wherein the insulating layer on the carrier element covers a larger surface than is required of the sealing layer.
 24. A sealing structure according to claim 1, wherein a first sealing device is arranged between the insulating layer and a neighboring separator plate, and a second sealing layer is arranged between the carrier layer and another neighboring separator plate.
 25. A method of producing a sealing structure for a fuel cell or an electrolyzer, comprising: producing at least one insulating layer on a carrier element; and producing at least one sealing layer made of a sealing material, the sealing structure being arranged in a sealing area of a fuel cell stack.
 26. A method according to claim 25, wherein the fuel cell or electrolyzer is one of a solid-oxide fuel cell and a solid-oxide electrolyzer.
 27. A method according to claim 25, wherein the insulating layer is produced by oxidizing the carrier element in at least one area.
 28. A method according to claim 27, wherein the oxidizing takes place at a temperature above 900° C.
 29. A method according to claim 27, wherein the oxidizing takes place at a temperature above 1,050° C.
 30. A method according to claim 25, wherein after the production of the insulating layer, the sealing layer is fitted on in the form of a sealing material strand.
 31. A method according to claim 25, wherein the sealing area is arranged between separator plates, and wherein producing at least one sealing layer made of a sealing material comprises: applying a sealing medium strand for forming a first sealing layer; fitting the carrier layer having the insulating layer onto the first sealing layer; and applying a sealing medium strand for forming the second sealing layer to the fitted-on carrier layer in the sealing areas.
 32. A fuel cell or electrolyzer comprising a sealing structure according to claim
 1. 33. The fuel cell or electrolyzer of claim 32, wherein the fuel cell or electrolyzer is one of a solid-oxide fuel cell and a solid-oxide electrolyzer. 